No period with Nexplanon. Now I have one why?

Hi... I got my implant in November 2015 my cycle stopped in February. Last Sunday I started spotting and then a light flow and still going on now it start out brown old build up blood and now red . Is this normal. Do I need to go the the doctor and get checked ? Please some one help... I did have unprotected sex a week prior to this could this mean a STD? Someone please help me.